Semiconductor wafers are frequently cleaned in cleaning solution into which megasonic energy is propagated. Megasonic cleaning systems, which operate at frequencies over twenty times higher than ultrasonic, safely and effectively remove particles from materials without the negative side effects associated with ultrasonic cleaning.
Megasonic energy cleaning apparatuses typically comprise a piezoelectric transducer coupled to a transmitter. The transducer is electrically excited such that it vibrates, and the transmitter transmits high frequency energy into liquid in a processing tank. The agitation of the cleaning fluid produced by the megasonic energy loosens particles on the semiconductor wafers. Contaminants are thus vibrated away from the surfaces of the wafer. In one arrangement, fluid enters the wet processing tank from the bottom of the tank and overflows the tank at the top. Contaminants may thus be removed from the tank through the overflow of the fluid and by quickly dumping the fluid.
As semiconductor wafers have increased in diameter, first at 200 mm and now at 300 mm, the option of cleaning one wafer at a time has become more desirable. A single large diameter wafer, having a multitude of devices on it, is more valuable than its smaller diameter counterpart. Larger diameter wafers therefore require greater care than that typically employed with batch cleaning of smaller wafers.
Verteq, Inc. of Santa Ana, Calif. has developed in recent years a megasonic cleaner in which an elongated probe is positioned in close proximity to the upper surface of a horizontally mounted wafer. Cleaning solution applied to the wafer produces a meniscus between the probe and the wafer. Megasonic energy applied to an end of the probe produces a series of vibrations of the probe along its length that are directed toward the wafer through the meniscus. Producing relative movement between the probe and the wafer, such as by rotating the wafer, has been found to be an effective way to loosen particles over the entire surface of the wafer, causing them to be washed away from the rotating wafer. An example of such an arrangement is illustrated in U.S. Pat. No. 6,140,744, assigned to Verteq, Inc., which is incorporated herein by reference.
Such a system provides very effective cleaning. However, as the height and density of deposition layers on wafers have increased, so has the fragility of such wafers. Current cleaning methods, including those using the device of the '744 patent, may result in damage to devices that are positioned upon the wafers. Such damage is, of course, a serious issue, because of the value of each wafer after layers of highly sophisticated devices have been deposited on the wafer. Thus, a need exists to improve the cleaning capability of such a megasonic probe system in a manner that will reduce the risk of damage to these delicate devices.
Through testing, Verteq, Inc. has determined that the extent of damage caused to each wafer is directly proportional to the power, or sonic watt density, applied to the probe. Damage can be reduced, then, by applying lower power. Testing has also shown, however, that reducing applied power may not be the best solution to the wafer damage problem, because reducing applied power may also decrease cleaning effectiveness. Further, simply reducing applied power in order to eliminate wafer damage creates a less versatile cleaning apparatus. For example, a probe assembly that can safely clean at applied power loads up to 50 W has a broader range of uses than a probe assembly that begins to damage wafers at only 15 W.
The most wafer damage results from waves that strike the wafer at a ninety-degree angle. But these waves do not necessarily provide the most effective cleaning. Waves that strike the wafer at angles more shallow than ninety-degrees do provide effective cleaning. Therefore, a modification to the device of the '744 patent that reduces the number of normal-incident waves without significantly reducing the number of more shallow waves would reduce the incidence of wafer damage without compromising the cleaning ability of the device.